The invention is based on a method for detecting errors in a motor vehicle cooling system. It is already known that the cooling-water temperature of a motor vehicle cooling system is controlled by opening and closing a thermostat valve. The temperature of cooling water is measured by a temperature sensor and fed to a computer that uses an algorithm and the measured values for the actual temperature to calculate a first temperature model band. By comparing the actual temperature with the first temperature model band, error detection is then carried out. Based on the result error message, however, it can be determined whether the thermostat valve or even the temperature sensor is defective. Nor is it possible to make a distinction between detects in the supply line or the display. On the other hand, there are legal requirements, in the United States, for example, which state that a defective cooling-water thermostat must be detected and displayed.
The method according to the invention for detecting errors in a motor vehicle engine cooling system has the advantage, however, that the individual trouble sources, such as a thermostat valve that does not close or a defective temperature sensor, can be detected selectively and displayed directly. This is achieved using the simulation of a second temperature model band that is calculated for the case in which a thermostat valve does not close. Using this simple method, detailed error detection can be carried out using a corresponding algorithm.
Advantageous further developments and improvements of the method indicated in the primary claim are possible using the measures listed in the dependent claims. It is particularly advantageous that the computer calculates the second temperature model band for a load-dependent temperature or different speeds. Based on the change of the course during the second temperature model band, a distinction can be made as to whether the thermostat valve actually no longer closes, or if there is a defect in the temperature sensor, such as an open circuit or an oscillation. By also taking the ambient temperature into account in particular when calculating the second temperature model band, the cause of the wrong indication can be advantageously determined with greater accuracy. This is an advantage in particular when further parameters such as the induction-air temperature, aspirated air mass, throttle-blade angle, and/or vehicle speed are included.
In order to be able to make an unequivocal claim about detecting errors in the cooling system, it is advantageous if the two temperature model bands are first analyzed when they no longer overlap.
On the other hand, a defective temperature sensor can be detected already if the actual temperature lies outside the two overlapping temperature bands for a specified period.
Since the second temperature model band is much flatter in shape than the first temperature model band due to the lower cooling-water temperature, a first unequivocal error diagnosis arises out of the course of the curve for the measured actual temperature.
The curve for the actual temperature is advantageously evaluated using a simple timer that tracks the actual temperature during a specified time interval. An error is present when the actual temperature lies outside the first temperature model band. If the course of the actual temperature actually lies outside both of the temperature model bands, it can be assumed that the temperature sensor is defective. If the curve for the actual temperature lies within the second temperature model band, however, this is an indication that the thermostat valve does not close, while the temperature sensor is in order. A possible cause can be, for example, that the valve is stuck in the opened state.